In order to be able to guarantee a highly reliable digital signal, readout electronics can be required to exhibit high initial accuracy, as well as high temperature and ageing stability. In addition, the high accuracy needs to be ensured over the whole specified temperature range and over a predefined time period, where the predefined time period may be the time between a manual calibration of the readout electronics and the next. During manual calibration, the manufacturer of the readout electronics applies an external standard reference voltage to the voltage signal input of the arrangement and uses the resulting digital signal to estimate and correct the gain error of the arrangement. The time period between two manual calibrations is extendable by enabling the arrangement for self-calibration. This is, for example, known under the term auto-calibration from the 3458A Multimeter by Agilent Technologies, as is described in the corresponding Calibration Manual, Manual Part Number 03458-90017. For the auto-calibration, the 3458A is equipped with internal reference standards, and the estimation and correction of gain errors is performed automatically whenever the auto-calibration function is invoked. During auto-calibration, the multimeter cannot be used for measurement purposes.
FIG. 1 shows a schematic diagram of known readout electronics, with a self-calibration circuit to correct the gain error of the readout electronics. The gain of the readout electronics is summarized by the series connection of a gain block GA and converting means ADC having a gain GB. The input signal to the readout electronics is, under normal working conditions, an analog voltage signal Vin, which is transformed into a corresponding digital output signal 2. For self-calibration purposes, internal reference means ref generate an analog reference voltage Vref, which is applied to an input switch 3 as well as to the converting means ADC. When self-calibration is desired, the input switch 3 needs to be operated in order to switch over from the analog voltage signal Vin to the reference voltage Vref. The digital output signal 2 of the converting means ADC is then expected to be equal to the overall gain of the readout electronics GA·GB, since the digital output signal 2 equals the digitized value of the analog input signal 1 divided by the digitized value of the reference voltage Vref, i.e. it equals to GA·GB·Vref/Vref. A discrepancy between the digital output signal 2 and the expected value GA·GB indicates the gain error of the readout electronics, the value of which is stored. Switch 3 is then operated back to apply the analog voltage signal Vin, and the gain error is used to correct the digital output signal 2 generated from the analog voltage signal Vin.
In the area of power transmission and distribution, voltages and currents can be measured by sensors for which it is desirable to perform self-calibration of the corresponding readout electronics without interrupting the current path of the analog voltage signal Vin, so that the availability of the readout electronics is increased and manual interaction with the sensors is reduced. This type of self-calibration can also be called online self-calibration.
Therefore, exemplary embodiments of the present disclosure provide an arrangement for reading out an analog voltage signal which, at the same time, delivers a signal which can be used to calibrate the arrangement without needing to disconnect the analog voltage signal.